1. Field of the Invention
This invention relates to a method of manufacturing a circuit module in which a bare chip or chips of IC, memory and the like and solder-connected electronic parts are mounted on a single electronic circuit board (hereinafter referred to as a substrate).
2. Description of the Prior Art
Recently, there have been increasing demands for ever smaller electronic equipments. Circuit modules which are to be built in electronic equipment are becoming smaller. Most ICs and memories are mounted on the circuit module in the form of a flat package called QFP or SOP. The flat package is formed by mounting a bare chip or chips on a lead frame, electrically connecting the lead frame with the bare chip and then molding the same with resin. However, from the viewpoint of aiming at smaller size of the circuit module, it is necessary to mount the bare chip directly on the substrate. Conventional examples of this kind of technology are disclosed in Japanese Patent Unexamined Publication No. 2-260592 and Japanese Utility Model Unexamined Publication No. 3-92046. FIG. 12 is a plan view of a conventional substrate, and FIG. 13 is a fragmentary enlarged sectional view showing a state in which cream solder is being applied onto the same substrate. FIGS. 12 and 13 simply illustrate the content of the above publications. In FIG. 12, reference numeral 1 denotes a substrate, and a large number of pads 3 are formed around a land 2 in the center of substrate 1. The pads 3 are formed in lines of four pads 3 so as to face to four sides of the land 2. A bare chip 4 is mounted on the land 2. Electrodes 5 on an upper surface of the bare chip 4 are connected with the pads 3 by means of wires 6. The bare chip 4 and the wires 6 are sealed with resin 7.
A large number of rectangular electronic parts 8 are mounted by the side of the bare chip 4. The rectangular electronic part 8 means that in which electrodes are formed on opposite sides of a box-shaped main body, examples of which are resistance chip, chip capacitor and so on. Electrodes 9 of each of the rectangular electronic parts 8 are fixed onto electrodes 10 of the substrate 1 with solder 11.
FIG. 13 shows a state in which, after mounting the bare chip 4 due to sealing with the resin 7, cream solder 11 is being applied onto the electrodes 10 by means of screen printing for the purpose of the soldering of the rectangular electronic parts 8. Reference numeral 12 denotes a screen mask in which pattern holes 13 are formed in the positions corresponding to the electrodes 10 of the printed substrate 1. The screen mask 12 is formed with a projecting portion 12a for avoiding the resin 7. As shown in the drawing, by sliding a squeegee 15 over the screen mask 12 put on top of the substrate 1, the cream solder 11 is applied onto the electrodes 10. In the succeeding step, reflowing is performed to cause the cream solder 11 to melt and solidify to form the solder 11 shown in FIG. 12.
However, in the conventional method described above, after the mounting of the bare chip 4, the cream solder 11 was applied by screen printing and then the rectangular electronic parts 8 were mounted, and therefore there was a problem that the rectangular electronic parts 8 had to be mounted in positions distant from the resin 7. Particularly as shown in FIG. 13, the screen mask 12 should be formed with the projecting portion 12a having a gently-inclined tapered surface a so as to allow the squeegee 15 to slide smoothly. For this reason, printing of the cream solder 11 was unstable in the vicinity of the tapered surface a, and therefore the pattern holes 13 were formed in the area at a horizontal distance L from the tapered surface a (or the area outside of the broken line in FIG. 12). In order to assure the distance L, the mounting positions of the rectangular electronic parts 8 had to be kept away from the bare chip 4 to a great extent, giving rise to a problem that the mounting of the bare chip could not contribute to smaller size of the circuit module so much.